If you can give an RPM rating at so and so voltage for the motor you're halfway there. You'll want to take into account if you're going to over volt and how that affects RPM, a bit into account obviously how the RPM is affected by load, wheel diameter, what top speed you want, etc.

(Or at least that's how the guys making go karts do it, it's the same principle for working out any gear ratio surely. Once you know wheel diameter and RPM, and the top speed you want, you can work out what you want. Probably best though to go by what the pros say, experience is often better than doing the math).

Just from the top of my head though, 100/60:1 is slightly insane. Unless of course you want to climb walls.